New --number-sections behavior from 3.1.12 increments counter in numbering even when .unnumbered class is used

[cderv]

Initially opened as a discussion at #9513

Pandoc 3.1.12 is changing the behavior of MakeSections (03bb426) and now .unnumbered section behaves differently. They are correctly not numbered, but they now increment the section number counter.

# pandoc 3.1.12
> pandoc --number-sections -t html
# One {-}

# Two

# Three {-}

# Four
^Z
<h1 class="unnumbered" id="one">One</h1>
<h1 data-number="2" id="two"><span
class="header-section-number">2</span> Two</h1>
<h1 class="unnumbered" id="three">Three</h1>
<h1 data-number="4" id="four"><span
class="header-section-number">4</span> Four</h1>

Previous behavior with 3.1.11.1 : Header with unnumbered class did not increment the section number and were skipped

# pandoc 3.1.11.1
> pandoc --number-sections -t html
# One {-}

# Two

# Three {-}

# Four
^Z
<h1 class="unnumbered" id="one">One</h1>
<h1 data-number="1" id="two"><span
class="header-section-number">1</span> Two</h1>
<h1 class="unnumbered" id="three">Three</h1>
<h1 data-number="2" id="four"><span
class="header-section-number">2</span> Four</h1>